As others have noted, borderless printing generally means that the printer will actually print up to 1/4 inch beyond the actual edge of the paper in order to avoid leaving a sliver of white if the paper is not perfectly aligned. If you have detail near the edge of the image it may be cropped or run off the photo.
The other thing that can happen is that the ink that is sprayed past the edge of the paper has to go somewhere and can build up in the printer. Sooner or later you may get streaks on the back of your photo or ever ink blotches on the edges. I usually try to use paper a little larger than the actual print, leave a border and trim the paper to size after printing. This adds another step to production, but I get my edges exactly the way I shot or cropped them and I do not get as much excess ink building up on the paper path of my printer.
Borderless printing is a great idea, but I no longer use it very often.